Supported silver catalysts have long been used in the conversion of ethylene and oxygen to ethylene oxide. The use of small amounts of the alkali metals, K, Rb and Cs, were noted as useful promoters in supported silver catalysts in U.S. Pat. Nos. 3,962,136, issued June 8, 1976 and 4,010,115, issued Mar. 1, 1977.
U.S. Pat. No. 3,844,981 issued Oct. 29, 1974, U.S. Pat. No. 3,962,285 issued June 8, 1976 and British Patent No. 1,325,715, published Aug. 8, 1973, disclose the use of silver-rhenium ethylene oxide catalysts. In these patents a high surface area silver derivative such as silver oxide is impregnated with a rhenium solution and subsequently reduced to provide metallic rhenium alloyed with the silver. The '285 patent discloses the use of KOH to precipitate Ag.sub.2 O from AgNO.sub.3. There is no disclosure in the patents of the use of suitable inert supports such as porous refractory supports. U.S. Pat. No. 4,584,921, issued Oct. 22, 1985, discloses the use of rhenium in silver-supported ethylene oxide catalysts. In this reference, the rhenium is first placed on the support in the form of finely divided metal particles and the silver is subsequently deposited on the outer surface of the particles. U.S. Pat. No. 3,316,279, issued Apr. 25, 1967, discloses the use of rhenium compounds, particularly ammonium and alkali metal perrhenate for the oxidation of olefins to olefin oxides. In this reference, however, the rhenium compounds are used, unsupported along with a reaction modifier (cyanides, pyridines or quinolines) in a liquid phase reaction. U.S. Pat. No. 3,972,829, issued Aug. 3, 1976, discloses a method for distributing catalytically active metallic components on supports using an impregnating solution of catalyst precursor compound and an organic thioacid or a mercaptocarboxylic acid. Catalytically active metals include metals of Groups IVA, IB, VIB, VIIB and VIII, including rhenium and which may be in either the oxidized or reduced state. However, promoting amounts of rhenium in combination with silver and promoter amounts of alkali metal on a porous refractory support are not suggested. U.S. Pat. No. 4,459,372, issued July 10, 1984, discloses the use of rhenium metal in combination with a surface metallated (using Ti, Zr, Hf, V, Sb, Pb, Ta, Nb, Ge and/or Si) alumina or silica. U.S. Pat. No. 4,005,049, issued Jan. 25, 1977, teaches the preparation of a silver/transition metal catalyst useful in oxidation reactions. In this instance, the silver serves as both a catalyst and a support for the transition metal co-catalyst. In U.S. Pat. No. 4,536,482, issued Aug. 20, 1985, catalytically active metals such as Ag and Re are co-sputtered along with a co-sputtered support material on a particular support. U.S. Pat. No. 3,449,078, issued June 10, 1969, discloses silver, alkali metal and rhenium, optionally sulfided, deposited on a carrier, preferably an alumina carrier.
Related applications are U.S. Ser. Nos. 926,025, now U.S. Pat. No. 4,766,105, and 926,026 now U.S. Pat. No. 4,761,394, both filed on Oct. 31, 1986 which relate to supported silver catalysts containing alkali metal and rhenium.
Commercial silver catalysts for the preparation of ethylene oxide are sold under the trade name Shell S809, S829 and S839. During application of these catalysts, one has observed that they becomes less stable, which means that the selectivity and the activity of the catalyst become lower and generally it is more economical to replace them by new silver catalysts after some years.
It is the object of the invention to find silver catalysts with improved stability.
Applicant has now found silver catalysts with improved stability.